Determination of oleuropein content, purification, and evaluation of In Vitro antioxidant and antidiabetic effects of leaves belonging to some olive varieties cultivated in our country

Objective: In this study, the amounts of oleuropein (OLP) and total phenolic content (TPC) in the leaves of olive varieties commonly cultivated in Turkey were determined. An efficient extraction and purification method for OLP was developed, and the antioxidant and antidiabetic effects of the obtained material were subsequently evaluated. Method: OLP extraction was performed using the ultrasonic-assisted extraction (UAE) method, and the process parameters were optimized using the Response Surface Methodology (RSM). The resulting extract was purified through the removal of lipophilic compounds, liquid-liquid extraction, and silica gel column chromatography. The OLP content was quantified using Ultra Performance Liquid Chromatography (UPLC), TPC was determined by the Folin-Ciocalteu method, antioxidant activities were assessed by DPPH● and ABTS●⁺ assays, and antidiabetic effects were evaluated using α-amylase and α-glucosidase enzyme inhibition tests. Results: Among the olive leaf varieties examined in this thesis, the Hamza Çelebi variety had the highest OLP content, while the Çilli variety had the highest TPC content. For the variety with high OLP levels, the optimal UAE conditions for maximum yield were determined as 87% ethanol, 60°C, and 37 minutes, resulting in an OLP yield of 4,87%. Following purification steps, OLP was isolated with 68,29% purity and a 74,37% recovery rate. The DPPH and ABTS activities of the purified material were 107,99±0,93 and 124,37±1,45 mg TE/g, respectively, while those of the extract were 56,21±1,28 and 43,23±1,23 mg TE/g. Both the extract (4,87% OLP) and the purified material (68,29% OLP) exhibited concentration-dependent inhibitory effects on α-glucosidase enzyme. In a comparative analysis, 2000 ppm of acarbose showed 79,38% inhibition, whereas the purified material and extract showed 56,55% and 58,65% inhibition, respectively. Conclusion: The findings revealed that both olive leaf extract and the purified material exhibited inhibitory activity on the α-glucosidase enzyme, comparable to acarbose, a commonly used antidiabetic agent. Keywords: oleuropein, ultrasonic, purification, antidiabetic, antioxidant